1. Field of the Invention
This invention relates to curable compositions comprising soluble oligomers having acryloyl groups and substituted methacrylate groups, and a process for preparing said oligomers. These oligomers may be self-crosslinked by radiation or crosslinked in reactions with other resins. The oligomers of this invention are useful as binders in curable inks and coatings.
2. Description of Related Art
Resins having acryloyl groups are widely used in industry, as for example as coating materials for paper, wood, metal and plastic, in printing inks, adhesives and sealants. The hardening of the materials having acryloyl groups is achieved by polymerization of the acryloyl groups with electron beam or with the help of a radical initiator. Furthermore, acrylates are able to crosslink with other reactive resins, such as unsaturated polyesters, polyacetoacetates or polyamines. The commercial production of resins, containing acryloyl groups, is performed by esterification of polyols with an excess of acrylic acid (Prepolymers and Reactive Diluents for UV- and EB-curable Formulations, P.K.T. Oldring (Ed.), SITA Technologies, London, UK, 1991, Page 124, 131).
However, the curing composition containing said esterification products containing acryloyl groups has a drawback which is lacking in hardness and in storage stability. Furthermore, though said esterification is widely used, this process bears several drawbacks. Acrylic acid, for example is rather unstable at elevated reaction temperatures and carries the risk of spontaneous uncontrolled polymerization, if not properly inhibited. The high viscosity of the prepared acrylate resins is another problem, aggravating the elimination of the excess of acrylic acid and the acidic catalyst after the reaction. Additional solvents are added to reduce the viscosity, so that the excess of acrylic acid as well as the acidic catalyst can be removed by repeated washing with alkaline aqueous solutions. The purification by distillation, often used in the case of low molecular monomeric acrylates, is not possible. The commercially used alternative method to obtain arcylated resins, the transesterification of polyols with monomeric short chain alkyl acrylates, exhibits, besides the above mentioned purification problems, also the problem of the separation of the formed alkyl alcohols during the reaction. These alkyl alcohols must be removed to proceed the reaction without removing the alkyl acrylates, which is difficult due to the small difference in boiling points, so that long and effective packed separation column are necessarily required.